Read disturb occurs when a read is performed on a word line in solid state storage. Cells on word lines (other than the one being read) may have a small amount of charge inadvertently added to them during the read; typically, cells storing the lowest voltages in unread word lines are affected the most.
One technique to mitigate read disturb is to have a counter for each word line which counts the number of reads since the last P/E cycle. As such, they are incremented with each read, and reset when the block is erased and re-written. These counters have a number of drawbacks associated with them. A first issue is size. Typically, the size of these counters is relatively large, for example 20 bits per counter. If there are 64,000 blocks and there is a counter per block, then this is over 1 MB of information which must be stored. This amount of overhead information is undesirable. A second issue is the type of storage in which these counters must be stored. The counts have to be preserved even if the system is power cycled, so non-volatile storage must be used. In some cases, this means periodically “flushing” the current counts from volatile storage to non-volatile storage. Even if a power down is indicated beforehand, there may not be enough time to flush the current counts to non-volatile storage (e.g., because the system has a maximum power down time). Finally, as a result of variations in manufacturing processing, some solid state storage devices are more durable than others. P/E counters do not take manufacturing variations into account. New techniques related to read disturb which address some or all of these issues would be desirable.